1. Field of the Invention
This invention relates to an electrolytic solution for electrolytic capacitor and an electrolytic capacitor.
2. Description of the Related Art
An electrolytic capacitor is generally manufactured as follows. Namely, an anode consisting of aluminum foil strip having on its surface an oxide film formed by anodic oxidation is superimposed with a separator sheet, a cathode plate made of aluminum foil strip and another separator paper sheet in the mentioned order thereby obtaining a laminate, which is then rolled thereby forming a rolled body. These separators are impregnated with an electrolytic solution to form a cathode, thereby accomplishing an electrolytic capacitor. The electrolytic solution functions through its inherent anodic oxidative effect to restore any defective portion of oxide film on the surface of the anode during the application of voltage between the anode and the cathode. It is required for the electrolytic solution to be low in specific resistance so as to prevent the electrolytic solution per se from excessively acting as a resistor. It is further required for the electrolytic solution to have a capability to form a high quality oxide film which is high in sparking voltage, i.e., high in withstand voltage in the occasion of restoring the defective portion of oxide film. Accordingly, if an oxide film which is low in sparking voltage is formed in the restoration of oxide film, usable voltage of a capacitor is limited. If this restored portion of oxide film is destroyed by the application of a voltage higher than this limited voltage, the capacitor becomes an unusable.
There are known various compositions of electrolytic solution for an electrolytic capacitor. For example, Jpn. Pat. Appln. KOKAI Publication No. 63-13321 discloses an electrolytic solution comprising a quaternary ammonium salt of cyanic acid dissolved in an organic solvent. This electrolytic solution, which comprises a quaternary ammonium salt of cyanic acid, is characterized as being low in specific resistance.
However, this electrolytic solution for an electrolytic capacitor is defective in that the sparking voltage thereof varies extensively due to the fluctuation of the content of water. The fluctuation of water content in the electrolytic solution may be caused by the water oozed out of the separator paper sheet during the operation of impregnating an electrolytic solution into the separator paper sheet wound together with an aluminum foil. Consequently, even if the water content in the electrolytic solution is adjusted before impregnation into the separator paper sheet, an accurate controlling of water content in the electrolytic solution after the assembling of the electrolytic capacitor becomes very difficult. This non-uniformity of sparking voltage resulting from the fluctuation of water content now causes a non-uniformity in characteristics of the electrolytic capacitor impregnated with the electrolytic solution, thereby badly affecting the reliability of a product.
Jpn. Pat. Appln. KOKAI Publication No. 2-63110 discloses an electrolytic solution for an electrolytic capacitor comprising a quaternary ammonium salt of aliphatic oxyacid dissolved in an organic solvent. This electrolytic solution comprising a quaternary ammonium salt of oxyacid is characterized as being low in specific resistance.
However, this electrolytic solution for an electrolytic capacitor is defective in that the sparking voltage thereof is low and unstable, the generation of sparking being concentrated at the interface between the electrolytic solution and air. Consequently, if this electrolytic solution for an electrolytic capacitor comprising a quaternary ammonium salt of oxyacid is to be served in practical use, the sparking voltage thereof has to be sufficiently raised and at the same time, stabilized by inhibiting the sparking from generating between the electrolytic solution and air.